Thursday, May 1, 2014

Immature "October Caddisfly" larvae, Dicosmoecus gilvipes

One of the popular hatches each year for fly fishermen in the West is that of the "October Caddis/Great Orange Caddis" that in Montana at least, normally takes place in September.  In my annual trips to Missoula, I've seen the occasional adult come off of the water and found mature larvae on the rocks in the rivers, but I've never been there when the full hatch is "on." (For photos of the adult see, for example,,

The cased caddisflies we found in the Washougal on Sunday (Camas, WA), were all immature Dicosmoecus larvae.  The mature larva makes a long, tubular case out of pebbles (~ 30 mm), but the immature larva makes a case out of a mixture of pebbles and sticks to which is attached pine needles.  Pretty clear example of that in the photo at the top of the page.

There is an interesting article on the function of those pine needles which I thought I would cite at the start of this entry.  "We investigated the functional role of cases built by Dicosmoecus gilvipes, a limnephilid caddisfly.  In this species, the 1st through 4th instar larvae build cases with plant material and attach Douglas-fir needles as lateral extensions that resemble vanes on an arrow.  We tested whether the lateral extensions and entire case deterred predators by manipulating lateral extensions and case presence for larvae exposed to large steelhead trout.  No larva with a case (with or without lateral extensions) was consumed during the experiment, whereas all larvae without a case were consumed."   (Michael P. Limm and Mary E. Power, "The caddisfly Dicosmoecus gilvipes: making a case for a functional role," Journal of the North American Benthological Society, 30(2): 485-492, 2011.)

Now to the diagnostic detail.  First from the Flyfishing Entomology website.  "Gills on ab segs w/2, 3, or 4 branches; femur on hind leg w/many long hairs; chloride epithelium (usually a shiny oval) on underside of 1st ab seg; case a long slightly curved untapered tube." (

1. Our larva does indeed have gills of 2, 3, and 4 branches on the abdominal segments.  You can see each type in the following photos, though the quality of my photos isn't the best.

2. And there are indeed many long hairs on the femur of the hind leg.

3. The chloride epithelium oval is not visible on this particular larva which is only 15 mm.  But it should be on segment 2, not 1 (see illustration on p. 215 of Wiggins).  However, I can show you what it will look like when these larvae mature using a larva collected a few years ago in Montana.

4.  We have an immature larva with a case of mixed pebbles and pieces of wood -- it's not yet an "untapered tube".  On the Discosmoeus case, Wiggins makes the following comments: "Final-instar larvae have a case of fine gravel, regular in outline, slightly curved, and often somewhat flattened.  In the early instars cases are largely of plant materials, and transitional example are often collected, demonstrating an abrupt change to gravel." (Larvae of the North American Caddisfly Genera, p. 214)  It's probably fair to call our cases "transitional" since they already have pebbles imbedded.

Other features used for ID are noted by Wiggins.  "These larvae are large and stout-bodied; sclerotized parts of the head and thorax are mostly uniform dark brown to black."  No question about that.

He continues, "there are metanotal setae on the membrane between the primary sclerites."  We can see them on our small larva,

but they show up much better when the larva's mature.

Also from Wiggins, "on segment IX a band of 20 to 40 setae extends ventrad from each side of the dorsal sclerite."

On our immature larva those setae are just barely visible,

much clearer when the larva's mature.

On the species ID, we go back to our website.  1) "Species atripes: Stout spines on front edge of pronotum; miniscule spines covering top of head and pronotum, giving them a fuzzy appearance." 2) "Species gilvipes: Stout spines on front edge of pronotum (particularly at the corners); absence of miniscule spines on top of head and pronotum." (   I'd go with D. gilvipes.  The spines on the corners of the pronotum are very clear: the head does not have a "fuzzy" appearance.


So there we have it.  Dicosmoecus gilvipes, the "Ocotober Caddis" at an early stage of the game.  Should get some photos of mature larvae when I go to Montana in August.  In the meantime, I can tell my son-in-law what to look for at the Washougal come September!  I imagine the steelhead will be running up the river before very long.

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